The present invention relates to an ultrasonic transducer and more particularly, to a capacitive ultrasonic transducer and a method of fabricating the same.
With the advantages of non-invasive evaluation, real-time response and portability, ultrasonic sensing devices have been widely used in medical, military and aerospace industries. For example, echographic systems or ultrasonic imaging systems are capable of obtaining information from surrounding means or from human body, based on the use of elastic waves at ultrasonic frequency. An ultrasonic transducer is often one of the important components in an ultrasonic sensing device. The majority of known ultrasonic transducers are realized by using piezoelectric ceramic. A piezoelectric transducer is generally used to obtain information from solid materials because the acoustic impedance of piezoelectric ceramic is of the same magnitude order as those of the solid materials. However, the piezoelectric transducer may not be ideal for obtaining information from fluids because of the great impedance mismatching between piezoelectric ceramic and fluids, for example, tissues of the human body. The piezoelectric transducer generally operates in a frequency band from 50 KHz (kilohertz) to 200 KHz. Furthermore, the piezoelectric transducer is generally fabricated in high-temperature processes and may not be ideal for integration with electronic circuits. In contrast, capacitive ultrasonic transducers may be manufactured in batch with standard integrated circuit (“IC”) processes and therefore are integrable with IC devices. Furthermore, capacitive ultrasonic transducers are capable of operating at a higher frequency band, from 200 KHz to 5 MHz (megahertz), than known piezoelectric transducers. Consequently, capacitive ultrasonic transducers have gradually taken the place of the piezoelectric transducers.
FIG. 1 is a schematic cross-sectional view of a capacitive ultrasonic transducer 10. Referring to FIG. 1, the capacitive ultrasonic transducer 10 includes a first electrode 11, a second electrode 12 formed on a membrane 13, an isolation layer 14 formed on the first electrode, and support sidewalls 15. A cavity 16 is defined by the first electrode 11, the membrane 13 and support sidewalls 15. When suitable AC or DC voltages are applied between the first electrode 11 and the second electrode 12, electrostatic forces cause the membrane 13 to oscillate and generate acoustic waves. The effective oscillating area of the conventional transducer 10 is the area defined by the first electrode 11 and second electrode 12. In this instance, the effective oscillating area is limited by the length of the second electrode 12 because the second electrode 12 is shorter than the first electrode 11. Furthermore, the membrane 13 is generally fabricated in a high-temperature process such as a conventional chemical vapor deposition (“CVD”) or low pressure chemical vapor deposition (“LPCVD”) process at a temperature ranging from approximately 400 to 800° C.
FIGS. 2A to 2D are cross-sectional diagrams illustrating a conventional method for fabricating a capacitive ultrasonic transducer. Referring to FIG. 2A, a silicon substrate 21 is provided, which is heavily doped with impurities in order to serve as an electrode. Next, a first nitride layer 22 and an amorphous silicon layer 23 are successively formed over the silicon substrate 21. The first nitride layer 22 functions to protect the silicon substrate 21. The amorphous silicon layer 23 is used as a sacrificial layer and will be removed in subsequent processes.
Referring to FIG. 2B, a patterned amorphous silicon layer 23′ is formed by patterning and etching the amorphous silicon layer 23, exposing portions of the first nitride layer 22. A second nitride layer 24 is then formed over the patterned sacrificial layer 23′, filling the exposed portions.
Referring to FIG. 2C, a patterned second nitride layer 24′ with openings 25 is formed by patterning and etching the second nitride layer 24, exposing portions of the patterned amorphous silicon layer 23′ through the openings 25. The patterned amorphous silicon layer 23′ is then removed by a selective etch.
Referring to FIG. 2D, a silicon oxide layer is deposited through the openings 25 to form plugs 26. Chambers 27 are thereby defined by the plugs 26, the patterned second nitride layer 24′ and the first nitride layer 22. A metal layer 28 is then formed over the patterned second nitride layer 24′ to serve as a second electrode.
In addition, conventional capacitive ultrasonic transducers usually include a silicon-based substrate. Conventional methods for fabricating such conductive ultrasonic transducers may use bulk micromachining or surface micromachining in a high-temperature process, adversely resulting in high residual stress, which may cause the deformation of the membrane of the capacitive ultrasonic transducer. To alleviate the residual stress, additional processes such as annealing may be required, which means a longer processing time and a higher manufacturing cost.
Furthermore, the chamber, or cavity, in a conventional capacitive ultrasonic transducer is generally formed by elements of different materials having different thermal coefficients, which may affect the performance of the transducer. Moreover, the membrane of a conventional capacitive ultrasonic transducer may be damaged when the transducer is assembled with a protection housing during package. It is desirable to have an improved capacitive ultrasonic transducer and a method of fabricating the same.